docs: def import_string(name):
      """ import module
      """
      mod = fp = None
  
      # code below, please refer to
      #   https://docs.python.org/2/library/imp.html
      # for details
      try:
          return sys.modules[name]
      except KeyError:
          pass
  
      try:
          fp, pathname, desc = imp.find_module(name)
          mod = imp.load_module(name, fp, pathname, desc)
      except ImportError:
          mod = None
      finally:
          # Since we may exit via an exception, close fp explicitly.
          if fp:
              fp.close()
  
      return mod